The present invention relates to a turbo vacuum pump in which pressure in an exhaust port is at or near the atmospheric level.
In recent years, due to the need for manufacturing miniature semiconductor elements and freedom from maintenance, to evacuate a filming apparatus such as the etching apparatus or chemical vapor deposition (CVD) apparatus a turbo vacuum is frequently used, in which magnetic bearings are provided to retain a rotor in a non-contact manner. Magnetic forces of electromagnets are used to float the rotor. An example of such a turbo vacuum pump is disclosed as a hybrid turbo molecular pump in Japanese Utility Model Laid-Open No. 127893/1992.
In this hybrid turbo molecular pump, a rotor having rotor vanes and screw grooves is rotatably housed in a casing having a suction port and an exhaust port, while stator vanes and a screw stator are mounted on the inner wall of the casing in positions confronting the rotor vanes and the screw grooves. Moreover, electromagnets are separately attached to the rotor, for generating radial and axial forces. The position of the rotor is detected by radial sensors and an axial sensor so that the exciting currents of the individual electromagnets are controlled on the basis of the outputs of those sensors to retain the rotor in a predetermined floating position.
The rotor thus magnetically levitated is rotated at a high speed. The rotor vanes and the screw grooves are rotated at a high speed relative to the stator vanes and the screw stator, to produce the pumping action so that the gas sucked from the suction port is compressed, and exhausted from the exhaust port. This exhaust port is connected to the suction side of a roughing vacuum pump such as an oil-sealed rotary pump or a dry pump using no oil in its working chamber.
When this pump is used in filming devices such as etching or CVD apparatus, a reaction product (e.g., aluminum chloride) may stick to or deposit on the inside of the pump. This deposit is is scraped off by detecting its amount and by the rotor rotating at varying angles within such a range that, if the detected deposit amount exceeds a predetermined value, the rotor and the stator are kept out of contact.
Another example of the turbo vacuum pump, in which the rotor is supported by the magnetic bearings so that the pump can be evacuated from the atmospheric pressure, is disclosed in Japanese Patent Laid-Open No. 101689/1994. In this example of the prior art, in a housing having a suction port and an exhaust port, there is disposed an exhaust pump unit having a peripheral flow pump stage and a centrifugal pump stage. A rotor having an integral construction of an exhaust pump unit and a motor for driving the exhaust pump unit is supported at its two end portions by magnetic bearings, and a seal is arranged between the suction port at the opposite side of the exhaust pump unit and the motor.
In the hybrid turbo molecular pump, as disclosed in the aforementioned Japanese Utility Model Laid-Open No. 127893/1992, when the deposition of the reaction product exceeds a predetermined value it is scraped off by rotating the rotor at an angle at a high speed, causing the reaction products which have stuck to and deposited on the rotor and stator, to collide with each other. As a result, a large sound or vibration may occur at an instant when the reaction products collide with each other. Moreover, the thin rotor vanes having a poor rigidity may warp so that, in the worst case of abnormal vibrations, if the balance is lost because of such warp, the rotor and the stator may contact and break, thereby impeding semiconductor manufacture.
Furthermore, since the hybrid turbo molecular pump cannot be operated at atmospheric pressure, it must be stopped when the vacuum chamber is returned to atmospheric pressure so that it may be cleaned. At the restart, the reaction products, deposited on the rotor or the stator, may congeal during cooling down, so that the hybrid turbo molecular pump is difficult to restart.
In the turbo vacuum pump disclosed in Japanese Patent Laid-Open No. 101689/1994, on the other hand, the vanes of the peripheral flow pump and the centrifugal pump are highly rigid, and the reaction product is constantly scraped off during the rotor rotation so that no special control is required. However, no consideration is given to restarting the pump after it is stopped, and the torque of the high-frequency motor for driving the pump rotor is insufficient to overcome the frictional resistance of the reaction product which has stuck to the rotor and the stator during the pump stop. Therefore, restart of the pump is difficult. If it fails to restart, the pump has to be overhauled, thus diminishing the maintenance free advantages obtained by using magnetic bearings.
An object of the present invention is to provide a turbo vacuum pump for exhausting from atmospheric pressure, which has magnetic bearings and can be restarted even if a reaction product sticks to its rotor and stator.
Another object of the invention is to provide a maintenance-free turbo vacuum pump to be exhausted from the atmospheric pressure.
A further object of the present invention is to provide an operating method capable of operating the vacuum pump maintenance-free.
Still another object of the present invention is to realize a highly reliable turbo vacuum pump with a long lifetime, that can exhaust from the atmospheric pressure, and a method of operating such a pump.
Still another object of the present invention is to provide a highly reliable turbo vacuum pump suited for exhausting a reaction product, and a method of operating the pump.
In order to achieve the above-specified objects, according to a first embodiment of the present invention, there is provided a turbo vacuum pump comprising: a housing having a suction port and an exhaust port; a stator retained in the housing; magnetic bearings mounted in the housing; a rotary shaft supported rotatably by the magnetic bearings; a drive unit for driving the rotary shaft; and a control unit for controlling the magnetic bearings, so that a gas sucked from the suction port is compressed and exhausted to the atmosphere from the exhaust port. A first sensor detects the RPM of the rotary shaft; and a second sensor detects either the electric current or the power of the drive means. The control unit includes a command unit for feeding the magnetic bearings with a command for displacing the rotary shaft by a predetermined amount if the detected value of the second sensor exceeds a predetermined value when the first sensor detects the rotary shaft is at a standstill.
According to a second embodiment of the invention, there is provided a turbo vacuum pump comprising: a housing having a suction port and an exhaust port; a stator retained in the housing; magnetic bearings mounted in the housing; a rotary shaft supported rotatably by the magnetic bearings; a drive unit for driving the rotary shaft; and a control unit for controlling the magnetic bearings, so that a gas sucked from the suction port is compressed and exhausted to the atmosphere from the exhaust port. A first sensor detects the RPM of the rotary shaft; a second sensor detects the acceleration time of the rotary shaft; and a controller controls the starts/stops of the magnetic bearing control unit and the drive unit on the basis of the detected values of the first and second sensors.
Preferably, the rotary shaft is provided with a centrifugal compression pump impeller and a multistage peripheral flow impeller sequentially from the side of the suction port, and the stator forms a multistage peripheral flow compression pump stage together with the peripheral flow impeller. Alternatively, the rotary shaft is provided with a screw slot pump impeller and a multistage peripheral flow impeller sequentially from the side of the suction port; and the stator forms a screw slot compression pump stage and a multistage peripheral flow compression pump stage together with the screw slot impeller and the peripheral flow impeller.
According to a third embodiment of the invention, there is provided a method of operating a turbo vacuum pump having a rotary shaft supported rotatably by magnetic bearings for exhausting to the atmosphere, in which the rotary shaft is displaced a predetermined number of times by the magnetic bearings before the turbo vacuum pump is rotationally started.
According to a fourth embodiment of the invention, there is provided a method of running a turbo vacuum pump having a rotary shaft supported rotatably by magnetic bearings for exhausting to the atmosphere. If either the motor current or consumed electric power exceeds a predetermined value at the time of starting the turbo vacuum pump, rotation is stopped and the rotary shaft is displaced a predetermined number of times by the magnetic bearings.
According to a fifth embodiment of the invention, there is provided a method of running a turbo vacuum pump having a rotary shaft supported rotatably by magnetic bearings for exhausting to the atmosphere. If at the time of starting the turbo vacuum pump the rate of increase of the RPM of the rotary shaft is below a predetermined value, rotation is stopped and the rotary shaft is displaced a predetermined number of times by the magnetic bearings. Moreover, in a preferred embodiment, after the rotary shaft has been displaced, a purge gas is fed from purge gas feed ports which are formed in bearing chambers for retaining the magnetic bearings, or the rotary shaft is displaced within a range in which it is kept out of engagement with touchdown bearings arranged in the vicinity of the magnetic bearings. Displacement of the rotary shaft may be a movement parallel to the center of the rotary shaft, a reciprocation along the rotary shaft, a conical motion on the axial center of the rotary shaft, or a conical motion on the vicinity of one end portion of the rotary shaft. Alternatively, the movement of the rotary shaft may be forward and reverse rotations through a small angle.
Moreover, if the rotary shaft acceleration is below a predetermined acceleration threshold, even when the movement of the rotary shaft is made a predetermined number of times, an audio or visual warning is given.
Preferably, moreover, a heating control unit heats the stator to a temperature ranging from 100.degree. C. to 180.degree. C. before a start of the turbo vacuum pump or after a stop of the rotation of the turbo vacuum pump.
According to a sixth embodiment of the invention, there is provided a turbo vacuum pump comprising: a housing having a suction port and an exhaust port; a stator retained in the housing; magnetic bearings mounted in the housing; a rotary shaft supported rotatably by the magnetic bearings; a drive unit for driving the rotary shaft; and a control unit for controlling the magnetic bearings, so that a gas sucked from the suction port is compressed and exhausted to the atmosphere from the exhaust port. The rotary shaft is provided with a centrifugal compression pump impeller and a multistage peripheral flow impeller sequentially from the side of the suction port, and the stator forms a multistage peripheral flow compression pump stage together with the peripheral flow impeller. A first sensor detects the RPM of the rotary shaft; and a second sensor detects an abnormality of the rotary shaft. A heating unit heats the stator, and a first controller displaces the rotary shaft by a predetermined amount on the basis of the detected values of the first and second sensors, while a second controller heats the stator to a temperature ranging from 100.degree. C. to 180.degree. C.
By means of the present invention, it is easily determined from the starting state of the rotary shaft that reaction product is stuck and deposited between the impeller and the stator of the turbo vacuum pump or between the shaft and the casing, and an overload upon the motor can thus be prevented to improve the reliability. If rotor lock due to the reaction product is confirmed at the start, the magnetic bearings are controlled to rock the pump rotor by the attractions of the magnetic bearings, so that the reaction product can be displaced. As a result, the pump can be restarted even if the torque of the drive motor is low. By comparing the motor current value, the RPM and the acceleration time at the start, moreover, a build up of reaction product is detected to displace the pump rotor so that the time period required for the control can be shortened. Since the pump rotor is displaced before the rotational start of the turbo vacuum pump, moreover, the starting time period can be shortened, and the rotational start can be ensured by removing the reaction product in such a deposition amount as will not lead to the rotor lock.
Even if the pump rotor is displaced to rock, moreover, the warning is made without accelerating the rotor so that the cause for a trouble can be clarified to cope with the trouble speedily. As a result, the time period, which seriously affects the semiconductor manufacturing process, is drastically shortened. When the rotor is not restarted even after if it has been displaced a set number of times, the stator is heated to a temperature lower than its heat treatment level (e.g., 180.degree. C.) and higher than the water evaporation level (e.g., 100.degree. C.), so that the solid aluminum chloride can be sublimated without lowering the performance of the vacuum pump. As a result, the pump can be restarted, and the performance drop which might otherwise be caused by the thermal deformation can be avoided because the heating temperature is lower than the level for the heat treatment of the stator material.
As a result, it is possible to provide a turbo vacuum pump for exhausting from the atmospheric pressure, which can be restarted even if the reaction product sticks during its stop, and which can exploit the advantage of the completely clean and maintenance free magnetic bearings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.